1. Field of the Invention
The invention relates, in general, to a bicycle frame that is aerodynamically shaped, lightweight, and stiff, including a main frame structure and front fork assembly, and in particular to the integral tension configuration, integral outer shell, integral tension struts, and integral tension ribs used in its construction.
2. Description of the Prior Art
Known prior art includes both traditional frame design, using traditional construction techniques and materials, and more recent innovative frame design, using new construction techniques and materials.
Traditional frame design and construction were developed under relatively limited availability of materials. As steel was readily available, cost effective, and relatively easy to form into simple structural shapes, round steel tubes were found to be the most efficient structural element to use in bicycle frame manufacturing. The construction technique used included the cutting and fitting of these tubes, and brazing them together at their joints with or without joint lugs.
Since traditional frame design, was developed primarily under the availability of round straight steel tubes, it primarily employed a two triangle design, with a rear triangle to carry rider load, and to hold the rear wheel, and a front or main triangle that also carried rider load and joined the rear triangle to the head tube and front fork thereof, and a front fork made of steel tubes. This was known as the safety bicycle.
From a structural standpoint the traditional two triangle design is essentially a very simple, short, open web truss. The top tube acts as a top boom, the down tube and rear wheel stays act as a bottom boom, and the seat tube and seat stays act as inclined interconnecting members between the top boom and the bottom boom, as in a typical open web truss of a bridge, for example.
A typical open truss is comprised of a top boom, a bottom boom, and interconnecting vertical and/or inclined members between the two. When a vertical load force is applied to such an open web truss, the top bottom is subjected to resultant compression forces, and the bottom boom is subjected to resultant tension forces, while the interconnecting members used to resist compression and sheer forces between the upper and lower booms may employ a combination of compression and tension members.
FIGS. 1A and 1B illustrate the similarities between the two structures by way of side view diagrams, and the directions of operative tension and compressive forces by arrows, with arrows pointing away from each other representing tension, and those point towards each other representing compression.
The simple open web truss that comprises the bicycle frame structure of the two triangle design is supported at each end with the axle of the wheels, in a way similar to a bridge truss abutment; indirectly through the front fork in the front end, and directly in the rear. When a rider load is applied to the top of the bicycle it causes the top tube and seat stays to go into compression, and the down tube and rear wheel stays to go into tension, while the seat tube, and seat stays act as inclined compression and shear resistant members.
The compressive and tensile strength characteristics of steel tubes, their availability and cost, and their workability, made them highly suitable for the two triangle design, and conversely made this design a very efficient and practical configuration, and most builders still use it with minor variations in the frame geometry.
Round steel tubes also work well to resist lateral and torsional flexes, and their ability to do so can improve by such things as adding flutes, internal rifling, double and triple butting, and increasing their diameter. Such increases in strength were sought to improve performance and allow weight reduction.
An essential structural feature of this design, however, is that it includes vertical and inclined members, and their postures limit their ability to receive significant aerodynamic improvement, even though attempts were made to do so by reducing frontal area, by using oval and tear drop tube shapes, reducing front wheel size, sloping the top tube, and so on.
So, even through the traditional two triangle design has desirable features in stiffness, weight, and vertical load bearing capability, its limitations in aerodynamics, as well as the need for speed in the area of competitive cycling, have driven on the search for more aerodynamically efficient configurations.
Other materials that have become more available, such as aluminum, titanium, and fiber reinforced composites, have provided builders with the opportunity to attempt new and innovative designs, that reduce frame weight and may offer significant improvements in aerodynamic efficiency.
While some bicycle frame builders have merely substituted tubes made of these materials for steel tubes, and gluing or welding of the joints in place of brazing in the traditional two triangle design, others have used new materials, in particular, fiber reinforced composites, to produce new bicycle frame designs which are aerodynamically far superior.
While some of these new frames have greatly improved aerodynamics with their streamlined shapes and efficient configurations, they have the reputation of being heavy, flexible, and/or bouncy, and thus are thought to have greatly reduced riding characteristics compared to traditional steel frames. One reason for this is that some of these frames, are, primarily, variants of the open web truss type construction, and employ traditional load bearing engineering principles. In addition, some of these innovative designs sometimes require complicated and costly construction techniques, as well as extensive mechanical adjustments. A superior design should address the aerodynamic efficiency, stiffness, strength, and weight requirements, of a bicycle frame simultaneously.